1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to improvement of the reliability of wire structures for connecting wires or conductors, which are formed on different layers, by using via holes.
2. Description of Related Art
In general, a metal wire in a semiconductor device is formed by patterning a metal thin film by a photolithography or etching technique. According to the type of each circuit, the thickness of the metal thin film ranges from about 300 nm to about 1000 nm, and the material used therefor is an aluminum alloy, titanium (Ti), titanium nitride (TiN) or other metal having a high melting point. The linewidth is determined by the amount of current flowing through the wire. The amount of current per unit sectional area of wire has been decided for each type of wire material to prevent problems attributable to electromigration, the established amount of current being known as permissible current density. For instance, when more current need to flow through a wire, a wider wire is used. Normally, a wider wire exhibits higher resistance to electromigration than a narrower wire; hence, a wire of a predetermined width or more is capable of retaining reliability even when the amount of current per unit sectional area is increased. For this reason, there is a trend toward increasing the permissible current density of a wide wire.
The recent trend toward multi-layer wiring requires that the reliability of wires be guaranteed in two or more layers electrically connected through via holes. This has led to a difficulty in increasing the permissible current density of the conventionally used wide wires, or it has even made it inevitable to decrease the permissible current density because a larger linewidth means a shorter electromigration life. This has resulted in an inconvenience in that the linewidth has been further increased, interfering with an attempt to make devices with even finer patterns or higher integration.
There has been another inconvenience in that, when electrical connection is made through a via hole between an upper layer wire and a lower layer wire orthogonally formed, concentration of current occurs around the via hole formed on the side of an interior angle of the orthogonal area, and voids due to electromigration tend to be produced in the neighborhood.